Measuring fluid flow velocity is useful for turbulence flow measurement in fluid mechanics research, and any industrial application where flow measurement is required, such as, but not limited to, gas metering and air duct monitoring. Commercial flow sensors are mainly based on one of two principles: thermal anemometry and laser-Doppler velocimetry. A thermal anemometer is a common type of commercial flow sensor for measuring the velocity of fluid flow. A typical type of thermal anemometer, often referred to as a “hot-wire anemometer”, utilizes a resistive heater (a “hot wire”) that serves as both a Joule heater and a temperature sensor. Monitoring the resistance of the resistive heater as current is passed through determines the temperature of the element.
Under a constant bias power and zero flow rate, the temperature of the resistive heater reaches a steady-state value. As flow of a fluid media passes the resistive heater, heat is transferred from the element to the fluid media via forced convection, thus reducing the temperature of the sensor. The flow speed is derived indirectly from the temperature variation from steady state values. Accordingly, the temperature of the resistive heater provides a means to gauge the cooling rate of the element and the flow velocity.
A conventional hot-wire anemometer includes a thin wire made of platinum or tungsten that is supported by prongs and mounted on a probe having a suitable electrical connection. This thermal sensor provides a fast response (in the kilohertz range), with low noise. The sensor also can be made relatively small and inexpensively.
However, conventional hot-wire anemometers suffer from significant shortcomings. One such shortcoming is that the fabrication process is delicate and may not result in uniform performance. Another problem is that it may be prohibitively difficult to form large arrays of the anemometers for measuring flow distribution, for example.
Micromachined anemometers have been used by those in the art to realize a thermal sensor with smaller dimensions, better uniformity, faster frequency response, and lower cost of production (via the batch processing nature of micromachining, for example). They also provide the ability to perform applications such as, but not limited to, flow field measurement. Conventional micromachined anemometers have been produced using a bulk micromachining technique, resulting in free-standing cantilevered structures within substrates. For example, doped polycrystalline silicon may be used to make prongs and resistive heaters by bulk micromachining. To create a significant distance between the resistive heater and the substrate, thus increasing thermal insulation to the substrate and increasing the sensitivity, the cantilevered structures are formed by at least partially removing the silicon substrate.
However, bulk micromachining incurs significant cost and restricts the type of substrate that can be used. For example, the doping of silicon (to create the resistive heater, for example), the etching of the silicon, and the packaging of individual silicon dies require significant expertise and effort. Additionally, most micromachined hot-wire anemometers cannot be realized effectively in a large array format. Furthermore, bulk micromachining requires significant etching time, and bulk etching using anisotropic wet etchants frequently poses concerns of materials compatibility, as all materials on a given substrate are required to sustain wet etching for long periods (several hours to etch through typical silicon wafers, for example).
Also, many other types of devices use doped polysilicon thin film as the material for the resistive heater. However, polysilicon deposition and annealing require a high-temperature process and generally preclude the use of substrates with a low melting point.
Certain microscale hot-wire anemometers use surface micromachining for a simpler fabrication process. However, the resistive heater employed is typically located directly on the substrate or very close to it. This leads to a slower frequency response and reduced sensitivity.